Item 31: Push Null Values to the Perimeter of Your Types

[Dan Vanderkam]

Item 31: Push Null Values to the Perimeter of Your Types

https://effectivetypescript.com/2020/03/24/null-values-to-perimeter/

[Dan Vanderkam]

Comment by Serhii on 2021-04-12 03:36:

From TypeScript perspective it is ok. But I strongly believe that it is not so good for V8 engine - https://v8.dev/blog/react-c... . Unfortunately, there is no good approach. The best way for V8 is to define result as NaN, but from other hand, TS is unable to distinguish NaN and number.

[Dan Vanderkam]

Comment by danvdk on 2021-04-12 09:19:

Interesting article. Reading it, I'd actually guess that pushing null values to the perimeter of your types would be a perf win, since it means no object shapes will ever change. Of course, be wary of speculation about performance wins and losses :) If you've got numbers on this, I'd love to see them.

[Dan Vanderkam]

Comment by Serhii on 2021-04-12 10:54:

I don't have numbers. But some time ago, I made small research in nodejs with help of V8 built ins. Every js object gets a shape (aka interface in TS). When you change the type of this object, V8 makes type transition under the hood. So if you have

let x = null; // HEAP_TYPE
x = 10; // SMI_TYPE

Here we had transition from HEAP_TYPE to SMI_TYPE. It is time consuming.
If you have a lot of transitions in your code - your code will be slower.

Even in this case:

let x = 1;
x = 1.1;

we have a transition from SMI_TYPE to DOUBLE_TYPE, it is also time consuming.
Please keep in mind, transitions are one directional.

When I say - time consuming, I mean it can slow down your code if you have billions of transitions. Of course it does not matter if you have several transitions.

Try to run this code:

class Foo {
    x = 40;
}

class Bar {
    x = 40.1;
}

const foo = new Foo();
foo.x = 43;
const bar = new Bar();
bar.x = 1;

Then, make Heap snapshot in devtools. Then you can find Foo instance and Bar instance. You will see that Bar Retained size is bigger. I believe it is because of transition from int to double

[Dan Vanderkam]

Comment by Serhii on 2021-04-12 10:56:

Btw, I learnt a lot from your book, it is a gem. Thanks you!